Looking at Wikipedia's DOT 3 page, it looks like it's been effectively superseded by DOT 4 fluid, but it's still sold and is still recommended on many forums. This question has an answer saying DOT 4 can be used in DOT 3 systems, but DOT 5 can't.

If DOT 4 is better, why is DOT 3 still in use? And the real question, which should I use on my bike that calls for DOT 3 (albiet, from 1983).

2 Answers
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The DOT3 standard has a lower minimum boiling point requirement then DOT4.

Not all fluids are made equal and they will all typically list what both their Dry and Wet boiling points are.

You can have one DOT3 fluid that just barely makes it past the standard, then another that can handle another 30-40F above that one.

If your bike calls for DOT3, then any DOT3 should be able to safely handle standard situations. Going with a higher quality DOT3 or a DOT4 will give you more tolerance though.

The Dry and Wet numbers are due to moisture build up in the brake lines. When you've first bled and put in new fluid, that fluid is operating in its "Dry" form. Over time, moisture will get into it which will lower its performance to the "Wet" level.

From what i learned dot 2 was suppoed to be drum brakes then the introduction of front disk brakes dot 3, and introduction of all wheel disks dot 4, and dot 5.1 late abs but what also happened is toyota went back to dot 3 possibly because of viscosity but havent researched it yet exactly.

I think so far the army is one of few using dot 5 but are thinking of going back to dot 3. This I cant back up yet so dont take my word for it.

Having stated this for example the nulon dot 3 has a boiling point of 260 C and states to not have adverse effect on physycal properties of rubber cups and o rings.

Im not a scientist and some manufacturers can and do make better fluids but I also found over time one will be good for a while then go slack on product.

Once I did get frustrated when i had to diagnose a fault on a car which had front wibration at 70kmh-80kmh then slightly less at 90-100. I did all usual check wheel alignment, balance tyres,check steering wrack, replace two lower ball joints, then overhaul callopers and finally found one cup swelled up in a master cylinder and would block a hole which would maintain pressure on front callopers causing front wheels to fight left each other. The fluid just came due for a flush and was dot 4 and Im avare its also true it can depend on car use and storage for fluid to need earlier flush.

Once I did a concentric clutch conversion on a car from ram clutches and ram cluch recomended only dot 3 stating other brake fluids can affect the rubber components.

So my conclusion is use the recomended dot but if neccessary can mix dot 3 and 4 only if last choice but then flush properly with correct fluid with two people not just gravity bleed because this stirs up everything in slaves and callopers and gives better indication when old fluid is flushed.